DAPPER defense lawyer Joe Tacopina has switched gears to work as a journalist for ABC News’ “20/20,” where his first story – about a sensational sex killing in Perugia, Italy – will air Friday. “It really is a fascinating whodunit,” said Tacopina about last fall’s slaying of British student Meredith Kercher during a drug-fueled orgy. Kercher’s roommate, Amanda Knox, is being held as a suspect, along with Knox’s Italian beau. Tacopina scored an exclusive interview with the chief of the Perugia police, got cameras into the Rome Crime Lab, and analyzed the murder scene and evidence. “I think this girl’s innocent,” he said of Knox.